Physical Investigation of Downburst Winds and Applicability to Full Scale Events

Thunderstorm winds, i.e. downbursts, are cold descending currents originating from cumulonimbus clouds which, upon the impingement on the ground, spread radially with high intensities. The downdraft phase of the storm and the subsequent radial outflow that is formed can cause major issues for aviation and immense damages to ground-mounted structures. Thunderstorm winds present characteristics completely different from the stationary Gaussian synoptic winds, which largely affect the mid-latitude areas of the globe in the form of extra-tropical cyclones. Downbursts are very localized winds in both space and time. It follows that their statistical investigation, by means of classical full scale anemometric recordings, is often inadequate in the view of accurately reconstruct the transient nature of the phenomenon. Wind tunnel tests in ad-hoc laboratories can fill this gap. Furthermore, downbursts never occur as isolated system in nature; they occur, in fact, embedded into the background Atmospheric Boundary Layer (ABL) flow and are influenced by the thunderstorm cell translation. In nature, the decomposition of the recorded downburst signals into component signals associated with the aforementioned contributions is often challenging or unfeasible. This study presents the results of the largest experimental campaign performed so far on downburst winds, where the physical behavior of downburst-like flows, simulated by means of the impinging jet technique, was thoroughly investigated in the spatiotemporal domain. The experiments were conducted in the Wind Engineering, Energy and Environment (WinEEE) Dome at Western University which allows the simultaneous generation of downburst and background ABL winds along with the simulation of the parent thunderstorm translation. For the first time, a clear understanding of the overall downburst dynamics and of the interactions that take place during the occurrence of the phenomenon is presented. Later, this study investigates, as a structural application, the aerodynamic behavior of two cylinders subject to the experimentally produced downburst winds at the WindEEE Dome. Finally, the thesis describes the vertical profile time-evolution of full-scale downburst events recorded by means of the state-of-the-art LiDAR profiler, installed within the large wind monitoring network developed along the northern Tyrrhenian coasts during the European Project “Wind and Ports”, with the aim of comparing the respective wind fields with those reproduced at the WindEEE Dome. Common characteristics concerning the transiency of the phenomenon in terms of mean and turbulent part of the wind speed signals are found and reported in statistical manner. It is found that the direction can be dealt as invariant with the height, the height of the maximum velocity drops in correspondence of the absolute peak velocity, and turbulence presents its maxima shortly before the occurrence of the peak velocity. The implications of these findings in terms of structural response can be crucial. This study is part of the wider project THUNDERR, whose Principal Investigator is Prof. Giovanni Solari, funded by an ERC Advanced Grant 2016. The project aims at finding a proper model of representation of thunderstorm winds, from the joint combination of physical, numerical, and analytical investigations, to be implemented in the calculation framework to assess the loading and response of structures to thunderstorm winds. The inclusion of an independent model for thunderstorm winds in the structural design codes, where the wind-structure interaction is still evaluated based on the synoptic-scale extra-tropical cyclones, would indeed represent a decisive turn. The problem is even more crucial in the view of the severe climate changes that are affecting the earth planet, which induce, as a consequence, a rising intensification and sharp increase in frequency of the extreme wind events, such as thunderstorms.